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HO-240-W
Commercial Greenhouse and Nursery Production
Purdue Horticulture and Landscape Architecture
www.ag.purdue.edu/HLA
Purdue Agronomy
www.ag.purdue.edu/AGRY
Purdue Floriculture
flowers.hort.purdue.edu
University of Kentucky Department of Plant and Soil Sciences
www.ca.uky.edu/pss
Soil pH
Michael V. Mickelbart and Kelly M. Stanton,
Purdue Horticulture and Landscape Architecture
James J. Camberato, Purdue Agronomy
Brad D. Lee, University of Kentucky Department of Plant and Soil Sciences
Plants obtain nearly all their nutrients from the soil, but not all the nutrients present in the soil
are accessible to plants. That’s because soil pH affects the form and, therefore, solubility of
nutrients (Figure 1). High soil pH will lead to deficiencies of micronutrients such as iron and
manganese, whereas low soil pH can lead to toxicities of certain elements such as aluminum.
This publication describes what soil pH is, how it is measured, and how it affects plants.
What Is Soil pH?
Soil pH is a measure of soil acidity or alkalinity.
The pH scale extends from 0 (a very strong acid)
to 14 (a very strong alkaline or base), but most
soils will have a pH range of 4 to 8.5. Pure water
is neither an acid nor a base and is considered
neutral, right in the middle of the scale at 7. pH
values less than 7 are called acidic and values
greater than 7 are called alkaline. If a soil has a
pH of 6, it is mildly acidic. If it has a pH of 8, it is
moderately alkaline.
The pH is the concentration (number)
of hydrogen ions (H+). It is calculated by the
following equation:
pH = -log 10[H+]
The more acidic a solution is, the higher the
concentration of H+ ions it contains. The pH scale
is logarithmic, which means that soil with a pH
value of 6 is 10 times more acidic than a soil with
a pH value of 7 and 100 times more acidic than a
soil with a pH value of 8.
How Is Soil pH Measured?
The pH of a soil solution can be measured with
pH meters, which range from expensive, very
accurate lab instruments to relatively cheap and
Figure 1. This graph shows how nutrient
availability changes with the pH of mineral
soils. Nutrients are most available when the
band is wide. When the band is narrow, the
nutrients are less available. Graphic adapted
from the Corn & Soybean Field Guide (Purdue
Extension publication ID-179). Source: Brady
and Weil, 2007.
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2
Figure 2. Severe interveinal chlorosis on leaves is a typical symptom of a micronutrient deficiency
caused by high-pH soils.
portable field-ready meters. A pH meter consists of
a glass electrode that changes its voltage output
in response to the pH of the solution in which it is
immersed. The instrument compares the changed
output to the constant voltage of a reference
electrode that is calibrated with solutions of known
pH values.
To use one of these meters, insert the probe
into a solution made from the soil sample — for
more information, see Commercial Greenhouse
Production: pH and Electrical Conductivity
Measurements in Soilless Substrates, Purdue
Extension publication HO-237-W, available from the
Education Store, www.the-education-store.com.
Dye-based methods are also available, but are less
precise. They are similar to the pH strips used to test
the pH of swimming pools or aquariums.
How and where the soil sample is obtained is
important. Soil pH can change with the horizontal
location in the landscape and soil depth. It’s best
to obtain soil samples from the rooting zone of
the plants of interest. For example, if you collect
a sample where turf grass is grown, collect the
soil from a shallower depth than you would for an
area where trees are grown. It is a good practice to
combine several soil samples from the planting area
to obtain a representative sample.
You can send soil samples to a commercial
laboratory to determine pH. Your Purdue Extension
county office can provide you with the name of a
reputable lab (to find your county office, visit www.
extension.purdue.edu/counties.html). Before sending
any sample, be sure to follow the lab’s sampling
instructions — improper sampling is often the
greatest source of error in soil testing.
What Is the Difference Between pH
and Buffer pH?
Soil pH is a measure of the concentration of
hydrogen ions in the soil solution. It describes the
acidity the roots experience.
Buffer pH (also referred to as reserve or stored
acidity) is a measures of the soil pH in a weak
base (pH 8 in some methods). Certain kinds of soil
particles tend to “store” acidity more than others —
that is, some soil particles have a reserve supply
of acidic ions bound to them. To change the soil pH
effectively, this reserve supply of acids bound in the
soil must be neutralized along with the free acids in
the soil.
The buffer pH value accounts for this difference.
Soils that have a lower buffer pH value require more
lime to neutralize the acidity than soils with a higher
buffer pH value. For example, a soil that has a buffer
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Soil pH
pH of 7.2 will require more lime to neutralize than a
soil with a buffer pH of 7.7.
How Does pH Affect Plants?
A soil’s pH is one of a number of environmental
conditions that affect the quality of plant growth.
Soil pH directly affects nutrient availability and can
influence plant growth (Figure 1).
Plants require 17 different nutrients to grow. Three
elements — nitrogen (N), phosphorus (P), and
potassium (K) — are required in comparatively
large amounts. They are called macronutrients and
are frequently applied as fertilizers. Plants require
micronutrients in smaller amounts but these nutrients
are just as vital to plant growth. When the supply
of any nutrient is less than the plant requires, plant
growth is inhibited, regardless of the abundance of
other nutrients.
Soil pH affects whether a given nutrient is more or
less available to the plant. Both too little and too
much of a nutrient can cause problems. Plants absorb
most nutrients from the soil through their roots. If
plants do not have access to the micronutrients they
need because the soil pH is high, their growth will be
suppressed and nutrient deficiency symptoms such
as chlorosis (yellowing) may appear (Figure 2).
When soils are acidic (have a low pH), some nutrients
are available in excess and plants take up more than
they need (often with toxic results).
Soil pH can also affect important microorganisms
in the soil, which in turn affects nutrient availability.
Plants cannot take up nutrients that are stored in
organic matter until the organic complexes are
broken down by microorganisms that help make more
nutrients available in a form that plants can take up.
Many of these microorganisms are most active when
the soil pH is around 8. When soil pH is less than
6, their activity is severely depressed. The natural
decomposition of organic matter is a reaction that
produces acid. That reaction can be accelerated by
tilling the soil.
Most nutrients are at their optimal availability to plants
when the soil pH is between 6 and 7. Many plants
grow best in soils in this pH range. However, there
are some plants (such as azaleas and blueberries)
that are adapted to growing in more acidic soils.
Factors Affecting Soil pH
In general, soils in natural settings have a pH ranging
from 4 to 8.5, depending on several environmental
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factors, including the soil parent materials from
which the soil was derived. Soils developed from
basic rocks (such as basalt or limestone) generally
have higher pH values than those formed from acidic
rocks (such as granite or shale).
Rain can also affect soil pH because it leaches basic
nutrients such as calcium and magnesium from the
soil, leaving behind acidic ions such as aluminum
and hydrogen in the soil in greater concentrations.
For this reason, soils formed under high rainfall
conditions (such as soils in Georgia), are more acidic
than those formed under arid (dry) conditions (such
as soils in Arizona).
Much of the soil parent material in Indiana is derived
from limestone or contains lime (calcium carbonate).
When this basic rock decomposes, it produces a
high pH (alkaline) soil. Fortunately, much of that
alkalinity has been reduced in the upper portion of
the soil because it has been leached out by rain.
However, the pH of much of the unweathered parent
material that is deep in the soil profile remains high.
Construction practices often bring these highly
alkaline soils to the surface, making the plant root
zone too alkaline for most plants to thrive.
Modifying Soil pH
You can modify soil pH to encourage plant growth.
In the soil, substances such as ammonium- and
urea-based fertilizers, organic matter, and elemental
sulfur undergo acid-producing reactions, which will
lower the soil pH. More information about strategies
for lowering soil pH are available in Commercial
Greenhouse and Nursery Production: Lowering
Soil pH for Horticultural Crops (Purdue Extension
publication HO-241-W, available from the Education
Store, www.the-education-store.com).
Adding lime, an alkaline material, will raise soil pH.
More information about liming to increase soil pH
can be found in Soil Acidity and Liming of Indiana
Soils (Purdue Extension publication AY-267-W).
Before modifying the pH of any soil, always consider
the optimum pH of the plants being grown, the
current pH of the soil, and any effects the potential
amendments could have beyond modifying the pH.
In some cases, it may be more practical to select
plants that grow well in the existing soil pH than
trying to change the soil pH.
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Soil pH
References
Brady, N.C. and R.R. Weil. 2007. The nature and
properties of soils. 14th ed. Prentice Hall, Upper
Saddle River, NJ.
Craul, P.J. and C.J. Klein. 1980. Characterization
of streetside soils of Syracuse, New York.
Metropolitan Tree Improvement Alliance Proc.
3:88-101.
Jones, J. 2001. Laboratory guide for conducting soil
tests and plant analysis. CRC Press, Boca Raton,
FL.
Lehmann, A. and K. Stahr. 2007. Nature and
significance of anthropogenic urban soils. J. Soils
Sediments. 7:247-260.
Marjan, K. and B. Lippert. 1999. Changing the pH of
your soil. Clemson University Extension publication
HGIC 1650.
Spies, C.D. and C.L. Harms. 1988. Soil acidity
and liming of Indiana soils. Purdue Extension
publication AY-267-W.
Tinus, R.W. 1980. Nature and management of soil pH
and salinity. Proceedings, North American Forest
Tree Nursery Soils Workshop, July 28-August 1,
Syracuse, NY. pp. 72-86.
To see other publications in this series, visit the Purdue Extension Education Store, www.the-education-store.com.
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Individuals using such products assume
responsibility for their use in accordance with current
directions of the manufacturer.
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